Two-Horned Oak Gall Wasp on the Peninsula: What Homeowners Should Know
Recognizing symptoms, understanding impacts, and knowing when treatment makes sense
Have you noticed Coast live oak leaves turning yellow, distorted, or taking on a burned appearance? Across the Bay Area, there has been an increase in activity from a pest called the Two-Horned Oak Gall Wasp (THOGW).
This insect feeds on the underside of leaves and causes swollen growths, or galls, to form. THOGW damage is identified by checking the underside of affected leaves for raised galls with two small horn-like bumps. Leaf symptoms can be mistaken for other issues, but the galls are a clear indicator.
What is the Two-Horned Oak Gall Wasp?
The Two-Horned Oak Gall Wasp (Dryocosmus dubiosus) is a gall-forming insect that can cause unusually severe leaf damage to Coast live oak (Quercus agrifolia). Heavy infestations have occurred across the region during the fall and winter of 2023/24, and activity is again elevated in 2025/26.
Gall wasps trigger plant tissue to form galls when they lay eggs on the tree. As larvae develop inside these growths, they draw resources from the tree and contribute to declining leaf health.
Gall wasps are considered cyclical pests:
- Activity varies from year to year.
- Regional activity has increased in recent years.
- Infestations are rarely fatal, but repeated or heavy attacks can contribute to stress, decline, and a thin, unhealthy appearance.
Common symptoms on Coast live oak
Infested trees may show:
- Yellowing and discoloration of leaves
- Distorted growth and a scorched or “burned” look
- Leaf necrosis
- Premature leaf drop
- Thinning of the canopy, particularly in the upper crown
Symptoms are often mild but can become more noticeable during heavier infestations or when trees are already stressed.
How THOGW affects Coast live oaks
Heavy infestations can:
- Deplete energy reserves and stunt growth
- Increase leaf loss and canopy thinning
- Stress and weaken the tree’s defense systems
A single infestation is usually non-fatal, but repeated infestations can accelerate decline.
When does treatment make sense?
In general, treatment for Two-Horned Oak Gall Wasp is not always necessary. Infested trees should first be inspected to identify underlying stressors and environmental conditions.
Treatment may be appropriate for stressed or declining Coast live oaks, or for trees that are important features in the landscape.
Treatment options used in plant health care programs
When treatment is recommended, options may include:
Non-toxic and cultural treatments
- Soil amendments and moisture management
- Fertilizer soil injections
- Horticultural oil canopy sprays*
- Cultural pest controls
Chemical treatments
- Systemic insecticides applied as soil drench, basal bark spray, or trunk injections
- Fertilizer soil injections
*Effectiveness of canopy sprays depends on infestation severity and available spray equipment. Chemical treatments have shown more rapid suppression of this pest.
About Ned Patchett Tree Care & Consulting
Ned Patchett Tree Care & Consulting helps homeowners and property managers throughout San Carlos and the greater Peninsula understand and manage their trees with clarity and confidence. If you’re ready to move past guesswork and myths and build a thoughtful plan for your trees, Ned Patchett Tree Care & Consulting can help you protect both your landscape and your peace of mind.
If you have questions about Two-Horned Oak Gall Wasp or would like to discuss treatment options for your Coast live oak, please email ned@nedpatchettconsulting.com or call (650) 728-8308.